To open the box remove the 4 rubber feet to gain access to the 4 screws beneith. Once removed the box opens easily.
The serial consol pads are clearly labled, Ground, Transmit and Recieve (GND, TX, RX)
As soon as my USB to TTL serial cable arrives ill look into modifing the U-Boot environment to boot a linux kernel from external USB storage or mmc card.
make ARCH=arm meson_reff16_defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- -j4
make uImage ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- -j4